Off-road motorcycles, also known as dirt bikes, specially designed for off-road use. The term off-road refers to driving surfaces that are not conventionally paved. These are rough surfaces, often created naturally, such as sand, gravel, a river, mud or snow. These types of terrain can sometimes only be travelled on with vehicles designed for off-road driving (such as SUVs, ATVs, snowmobiles and mountain bikes in recent decades, and minibikes even earlier) or vehicles are designed to better handle off-road conditions. Compared to road-going motorcycles, off-road machines are lighter and more flexible, typically having long suspension travel, high ground clearance, and are geared higher to provide more torque in off-road situations. Wheels (usually 21″ front, 18″ rear) have knobby tires, often clamped to the rim with a rim lock. Many competitive events have emerged[when?] and developed into a variety of off-road motorcycle sports, for which a number of specialized motorcycles have been built: Motocross—Such bikes are raced on short, closed off-road tracks with a variety of obstacles. The motorcycles have a small fuel tank for lightness and compactness. Long-travel suspension allows riders to take jumps at high speed. Motocross engines are single-cylinder two-stroke or four-stroke units, which vary in size from 50cc up to about 500cc. At the professional level, bikes are split up into two levels based on their displacements: MX and MX Lite. The MX Lite class contains 125cc two-stroke engines and 250cc four-stroke engines, while the MX class pits 250cc two-stroke engines against 450cc four-stroke engines. The differences in power, displacement, torque, and weight are all variables that balance the competition between two-stroke and four-stroke engines. Motocross sidecar outfits have bigger engines, usually four-stroke and often twin-cylinder. Motocross bikes are also used in freestyle motocross. Enduro—A modified and road-legal motocross bike, having the addition of a horn, lights, effective silencing and a number plate. Enduro riders compete over a longer course (which may include roads); and an enduro event may last between one day and six days (such as the International Six Days Enduro). Some enduro events (known as “multi-lappers”) are held on rather shorter circuits, not unlike scramble tracks. “Multi-lappers” are especially popular with novice riders. Rally raid, or “rallies”—A special type of enduro bike with a significantly larger fuel tank for very long distance racing, typically through deserts (e.g. Paris-Dakar rally). Engine capacities tend to be larger, usually between 450 cc and 750 cc. Dual-Sport—A dual-sport bike is a multi-purpose bike, made for on-road and recreational off-road riding. A dual-sport bike may resemble an enduro bike, but since a dual-sport bike is not intended to be used for competition, it may be less rugged, and equipped with dual-purpose tires and with more road legal equipment, such as indicators, mirrors and extra instruments. Most dual-sport bikes require a number plate to be ridden on state and county roads. Trials—Trials riding is a specialized form of off-road competition testing balancing skills and precision rather than speed. For a trials bike, low weight and crisp throttle response power are prioritized, so trials bikes tend to have a small (125 cc to 300 cc) engine, with two-strokes being common. During the trial, the rider stands on the foot-pegs, so a trials bike will have only a vestigial seat, or no seat at all. Fuel tanks are very small, giving a very limited range. Track racing—High-speed oval racing, typically with no brakes, nor rear suspension. The engines, fueled by methanol, are long-stroke four-stroke singles, such as JAP and Jawa. They have at most two gears. Some types, such as speedway, and grass-track bikes, are designed to take left turns only. Snow bikes—A snow bike takes a typical dirt-bike and replaces the rear wheel with a single tread system similar to a snowmobile and the front wheel with a large ski. They are much smaller and more nimble than a snowmobile, and they have a tighter turning radius, which lets the rider go where many snowmobiles cannot. The first prototype of motorcycles with a rear tread date all the way back to the 1920s, with failed attempts to bring them onto the market until recent times. Many motorcycles made after the 1990s or later can be fitted with a kit that transforms them into a snow bike.